Not all Cushitic people were nomads . Ehret and Blenchs reconstruction of Cushitic farming lexicon shows that farming vocab was native to the cushites. In fact , it was not uncommon for people to practice a lifestyle combining some form of agriculture and pastoralism . The Oromo groups are the best example of this . The Madanle Azanian people of Southern Somalia also practiced this. There are some climatic reasons why some groups went from farming to nomadism .Cushitic people were nomads, not farmers. They didn't settle in the Nile River, and if they did then they wouldn't have migrated South into the Ethiopian highlands(where they then settled) and further south all the way to Tanzania. The Beja are the only Cushitic people(also nomads) who live in the Red Sea region between Egypt-Sudan which was the ancient Cushitic migratory path.
As for the Nubians , they were not exactly killed off and enslaved . The Muslim army failed to defeat them
And signed a peace treaty with them lasting many centuries . The Arab descendants became a majority due to migration , trade and some element of warfare (the Afro centrics only concentrate on this aspect ).